A CLOSELY-SPACED MAGNETO TELLURIC STUDY OF THE AHUACHAPAN-CHIPILAPA GEOTHERMAL-FIELD, EL SALVADOR

The distribution of electrical conductivity beneath the Ahuachapan-Chi pilapa geothermal area was simulated using 2-D models based on 126 clo sely-spaced magnetotelluric (MT) measurements. The observed MT respons e was interpreted as being produced by the superposition of two orthog onal geological structural systems: an approximately E-W regional tren d associated with the Central Graben structure, which affects the long er period response, and a local and younger N-S fault system that is r esponsible for the short-to-intermediate period data. The MT response in the 0.02-10 s range period was used to simulate the conductivity st ructure within the first 2 km depth. By correlating the low-resistivit y zones between twelve 2-D models, maps of the spatial distribution of conductors at three different depth levels were constructed. Three de ep conductors were identified, one of them associated with the Ahuacha pan reservoir, another apparently related to the Laguna Verde volcano, and a third one controlled by EI Tortuguero Graben. The subsurface ge ometry of these conductivity anomalies suggests that the Chipilapa and La Labor hot springs are supplied by two separate sources of hot flui ds, one coming from the east and the other from the south or southwest . The distribution of the shallow high-conductivity zones agrees with the hydrothermal alteration zones mapped at the surface, suggesting th at at shallow levels the argillitization process contributes significa ntly to the low resistivity. The large number of drillholes and the de nse MT site coverage allowed the definition of important correlations between high temperature and high conductivity, as well as between dee p conductivity anomalies and productive wells. On this basis two areas for future drilling are proposed. (C) 1997 CNR. Published by Elsevier Science Ltd.